. Pneumocystis carinii is the leading cause of morbidity and mortality in patients with AIDS, where it causes life threatening pneumonia. P. carinii preferentially colonizes the lungs, attaching to alveolar epithelial cells (e.g. type 1 pneumocytes). Little is known regarding the molecular mechanisms underlying the interaction of the parasite and the lung. Adherence by P. carinii appears to be critical to initiate infection. P. carinii ligands that have been reported to be involved in cell binding include: glycoprotein 120 (gpA), a high molecular weight protein that binds vitronectin, and mucin-binding lectin. The role of sialic acids and sialic-acid -binding proteins (i.e, trans-sialidase) have been recently investigated in the pathogenesis of Trypanosoma cruzi and are now identified in P. carinii. The novel class of enzymes trans-sialidase have been identified in a few pathogenic protozoan parasites (e.g., T. cruzi) and appear to promote adhesion to host cells by reacting with sialyl receptors. The P. carinii protein has been purified by this research group and has been partially biochemically defined. Monoclonal and polyclonal antibodies cross reactive with the P. carinii protein block parasite adhesion to lung alveolar cells, suggesting that the trans- sialidase is a mediator of P. carinii- host cell interaction. The goal of this proposal is to further extend these exploratory experiments to gain a better understanding about the biochemistry and cell biology, as well as the possible clinical relevance of the trans sialidase. Proposed in situ and experimental animal studies of P. carinii infection will be used to assess the biochemistry of trans-sialidase in P. carinii lung colonization. Specific objectives include: 1) Ascertain the acceptor-substrate specificity of trans-sialidase; 2) Produce MAbs against trans-sialidase; 3) Localize sialyl residues and trans-sialidase on P.carinii ultrastructurally; 4) Demonstrate sialic acid receptors on P. carinii; 5) Clone the trans-sialidase gene; 6) Ascertain whether host cell sialic acid is a receptor for P. carinii attachment to lung epithelium; 7) Determine trans- sialidase pathway in P. carinii adherence; 8) Test effectiveness of humoral response to the trans- sialidase against P. carinii infection.